Field of the Invention
The present invention relates generally to electronic packaging, and more particularly to water resistant surface mount packaging having multiple emitters for outdoor displays.
Description of the Related Art
Light emitting diodes (LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
In recent years, there have been dramatic improvements in LED technology such that LEDs with increased brightness and color fidelity, smaller footprints, and overall improved emitting efficiency have been introduced. LEDs also have an increased operation lifetime compared to other emitters. For example, the operational lifetime of an LED can be over 50,000 hours, while the operational lifetime of an incandescent bulb is approximately 2,000 hours. LEDs can also be more resilient than other light sources while consuming less power. For these and other reasons, LEDs are becoming more popular; they are being used in more and more applications as an alternative to incandescent, fluorescent, halogen and other emitters.
Due at least in part to these improved LEDs and improved image processing technology, LEDs can be used as the light source for a variety of display types. In order to use LED chips in these types of applications, it is known in the art to enclose one or more LED chips in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. The LED packages are commonly mounted on a printed circuit board (PCB).
Different LED packages can be used as the light source for outdoor displays of varying sizes, and are expected to withstand a variety of environmental conditions. These LED-based displays can comprise thousands of “pixels” or “pixel modules”, each of which can contain a plurality of LEDs. The pixel modules can use high efficiency and high brightness LEDs that allow the displays to be visible from various distances and in a variety of environmental lighting conditions. The pixel modules can have as few as three or four LEDs that allow the pixel to emit many different colors of light from combinations of red, green, blue, and/or sometimes yellow light.
Most conventional LED based displays are controlled by a computer system that accepts an incoming signal (e.g. TV signal). Based on the color needed at the pixel module to form the overall display image, the computer system determines which LED(s) in each pixel module is to emit light, and how brightly. A power system can also be included that energizes each pixel module; the power to each of the LEDs can be modulated so light is emitted at the desired brightness. Conductors are provided to apply the appropriate power signal to each of the LEDs in the pixel modules.
LED-based displays are becoming more common in outdoor applications, and as their popularity increases, it is important for LED packages incorporated into such displays to withstand potentially harsh environmental conditions. The packages are expected to be robust and maintain their structural integrity during manufacture and use, but the integrity of such packages can become compromised. For example, in current packages in the art used for outdoor displays, separation between the package casing and the leads/lead frame can occur. Such separation can allow environmental contaminants such as water and dirt to enter the package, and can reduce the overall life of the package. Furthermore, the package structures can become deformed during the reflow process.